[gcc r15-1450] libstdc++: Add conditional noexcept to std::pair default ctor

[Jonathan Wakely]

https://gcc.gnu.org/g:9651d6005f9c1ac60aecf7b36d6c0bd1ead8a63b

commit r15-1450-g9651d6005f9c1ac60aecf7b36d6c0bd1ead8a63b
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Jun 18 20:57:24 2024 +0100

    libstdc++: Add conditional noexcept to std::pair default ctor
    
    Most of std::pair constructors implemented using C++20 concepts have a
    conditional noexcept-specifier, but the default constructor doesn't.
    This fixes that.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/stl_pair.h [__cpp_lib_concepts] (pair()): Add
            conditional noexcept.

Diff:
---
 libstdc++-v3/include/bits/stl_pair.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/libstdc++-v3/include/bits/stl_pair.h b/libstdc++-v3/include/bits/stl_pair.h
index 0c1e5719a1a3..0d60eaba1941 100644
--- a/libstdc++-v3/include/bits/stl_pair.h
+++ b/libstdc++-v3/include/bits/stl_pair.h
@@ -344,6 +344,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       explicit(__not_<__and_<__is_implicitly_default_constructible<_T1>,
 			     __is_implicitly_default_constructible<_T2>>>())
       pair()
+      noexcept(is_nothrow_default_constructible_v<_T1>
+		&& is_nothrow_default_constructible_v<_T2>)
       requires is_default_constructible_v<_T1>
 	       && is_default_constructible_v<_T2>
       : first(), second()